text:
Do aircraft have accelerometers?
Do modern aircraft employ accelerometers or other similar equipment to measure g-forces on the aircraft? I could imagine this being used as an inspection/service indicator, particularly after hard landings. Or do hard landings and other forceful flight regimes typically not pose any significant structural concerns?
Update. Related on determining hard landing inspections:
From Aero Magazine on Boeing conditional inspections
Section on AMM changes:

Boeing is modifying the 737 AMM, section 05-51, and will revise the AMMs for other Boeing-designed airplane models, to include these vertical acceleration values (table 1). For Douglas-designed airplanes, similar values are being generated and will be available in the AMMs in early 2001. The values are intended as thresholds that can be used to help determine whether a hard landing inspection is necessary. If the flight crew concludes that it has experienced a hard landing, the AMM conditional inspections should be performed even if the acceleration readings do not exceed the values added to the AMM. The vertical acceleration values are to be used by operators, in addition to or in lieu of flight crew judgment, to initiate conditional maintenance inspections.

Source: Aero Magazine

Given many FBW laws are based on G forces, it would be astonishing if the F16, the A320, ... were not fitted with accelerometer.

Accelerometers are the basis of modern rate gyros which are widely used.

@PeterSmith Don't you mean accelerometers are the half the basis of intertial measurement units? The other half being rate gyros. Unless you consider rate gyros using the of coriolis acceleration to be a form of acceleroemter.

OP, the simple answer to your question is "Yes, of course, ubiquitously".  OP, you seem to focus particularly on hard-landing-stuff.  Even completely setting aside that issue, the answer is simply "yes, sure".

A smarty pants answer here would be "assuming humans are on board, yes".  (Every human has at least one phone; every phone has many accelerometers.)

Most modern aircraft, which includes long range airliners since around 1970, all airliners since not much later, and basically anything with glass cockpit, do have very accurate accelerometers for all three axis, as part of the inertial reference system.
They are important instruments for the autopilot, as they provide faster feedback on the effect of control deflections than the air-data references, and essential for navigation as cross-reference for detecting GPS errors and the inertial navigation serves as a backup for GPS (and served as primary navigation over the oceans before wide GPS availability). Airbus (since A320) even defines the pitch command by vertical acceleration in the normal law. And yes, they are also used for detecting hard landings and their severity.
The pilot never sees the direct accelerometer output except in fighter and aerobatic aircraft though (well, almost never; there are a couple of other cases).

By the way, inertial systems also require gyroscopes or angular rate sensors besides accelerometers, and their data needs to be integrated and fused with an external reference for long-term accuracy, hence they usually serve integrated with GPS, and for a while the inertial system allows navigating during GPS outages.

@JanHudec: You are correct, I am confusing the mechanism used in mems to detect rate from the Coriolis force. Not relevant to the measurement. Let me delete my previous misleading remarks.

@copper.hat, on a side note, the linear acceleration can, and used to be, measured using gyroscopes. If you mount a gyroscope so it can only rotate around one axis, it will precess due to linear acceleration and it's orientation will correspond to the integrated linear velocity. The early inertial navigation systems did it that way.

@JanHudec: I suspect you already know this, but in a vaguely similar manner the mems gyros. have two 'counter' oscillating masses that respond to rotation, the Coriolis force changes the capacitance between them which can be measured.

Well I dunno,  some aircraft are intended to be capable of doing the occasional aerobatic maneuver on the side, and thus have g-meters, even though aerobatic flight is not their primary mission.  E.g. most Schweizer 2-32 sailplanes.  Is this an "aerobatic aircraft"?  Maybe...

I think you need to define "modern" aircraft, the question is pretty broad as-is.  And even interpretation of what you mean could vary.  i.e. are you referring to a real time dial showing actual Gs in the cockpit?
As quiet flyer noted, as a reference instrument an accelerometer is very important for aerobatic flight.
However, while a real time instrument is not needed for a larger air carrier, accelerometers feeding data to a flight data recorder for mishap investigation and recording of life cycle data for airframe fatigue calculations is important.
Most small modern general aviation aircraft don't have them though.

Any plane with an AHRS system will most likely employ accelorometers in the attitude solution.

Good point.  Highlights my point though, is the question about a cockpit gauge or a stress force data recorder?  Besides attitude, what might an AHRS accelerometer output?  Will a G-1000 record a structural overstress?

Technically, "accelerometer" is the sensor itself, and this is how I would interpret the question. In this case, most but the simplest aircraft have them, primarily as a part of automatic/augmented control and/or navigation systems.

@Zeus, that's one interpretation, but since the question specifically mentioned measuring G forces, and the "meter" in accelerometer causes me to visualize the cockpit gauge I am familiar with, I interpreted it differently.  So, would a sensor that feeds a navigation system common to GA record G forces in a way that can be read by the pilot and/or maintenance personnel?

Why GA? The question is not limited to GA; if anything, a random person would more likely think of airliners by default. But even in GA, those airplanes that are equipped with AHRS (e.g. G1000 suite), can display a technical page with all accelerations. But it is true that pilots don't normally refer to acceleration/G force (except for aerobatics/fighters), so it is not common to have the indicator in the cockpit.

My Dynon Skyview EFIS uses the accelerometer data from its AHRS to pop up a G meter display when load exceeds a configurable level (1.6 g by default, IIRC) and gives an audible warning "geez!" if you exceed another configurable threshold. This data is also logged.

@pericynthion, that's pretty cool, I didn't know that!  Does anybody happen to know if that sort of feature is available in the GTN-650?

@Zeus, why should either one of us make any assumptions about the OP's intent or what a random person might think?! All the question said was "modern", and in my answer I critiqued that. And I didn't limit my answer to GA, I broke it down between an aerobatic plane that needs real time Gs, to an airliner that needs stress data, to a (round dial) GA aircraft that doesn't have it. If you have a better answer please post it and let's compete for upvotes.

@michaelhall I don't think the GTN-650 even has any connection to an AHRS

+1 for airframe fatigue as structural concerns are present in the question.

Interesting points in the comments. The question mainly concerned structural overstress in airliners, with for instance an alert to crew or maintenance that a given incident went beyond a set threshold. That being said, additional information about navigation and gauges in other types of aircraft, GA and aerobatics / fighters is also welcome

The Kanardia Horis EFIS installed in my glider displays g force on an on-screen meter that is too small to practically use for aerobatics (though any manoeuvrers which seriously required one aren't really suitable for the aircraft, which has an airworthiness category of "semi-aerobatic").
The acceleration sensors are essential for the unit to work as an artificial horizon for cloud flying, but the real time g display is essentially a gimmick, and frankly if you lost it in cloud, you'd rather not know what g you were pulling!

@Erik, we know that now, but when you just say "modern" you open things up for interpretation.  And as you can see, pilots and engineers think differently!

@MichaelHall Indeed they do - I will keep that in mind when I post next time. Thanks for your answer and comments

When I worked in the industry in the early 2000s, Eurofighters had g-sensors recording during flight, and put you into a different engine service regime above a certain limit.  There was a matching detent on the stick (not sure if virtual or physical) that warned pilots if they were going to go over the limit.
Twenty years on, given the cheapness of modern mems sensors, I'd find it unlikely that any large aircraft engine lacked any accelerometers, though in most cases they are more likely to be used to monitor vibration or fatigue life than g load. A quick search finds many suppliers, e.g. Meggitt, who list the following uses, but not specific engines or airframes they are used on:

Our piezo-electric accelerometers are ideally suited to a wide range of jet engine (civil and military), helicopter and space applications:

gas turbine engine monitoring
airframe structure vibration analysis
gearbox analysis
bearing analysis [etc]

Eurofighters are rather different than the Cessnas, Pipers, and such that I expect most of us fly, and both are different from commercial airliners.

Depends on the aircraft in question.  Airplanes used for aerobatics or military fighters will commonly carry them as a reference for pilots on just how hard they’re loading the airplane at any one time, though many good aerobatic pilots have enough time that they can kinesthetically sense just how hard they’re loading the airplane.
Transport category aircraft don’t carry G meters as it’s not very useful to their flight profiles.  Hard landings are gauged kinesthetically and by the vertical speed at touchdown.  Exceed certain metrics and the airplane will be inspected by protocol.

I suspect that Gz is part of the automatic hard-landing detection support on some airliners (I believe Airbii will spit out a report automatically if you land hard enough)

Of course they do. You can find them in just about any cockpit from the modern aircraft of today down to about the 1930's. Here's an example.

Strange.  I'll grant that my experience is fairly limited, but I've never seen one.  And why is the one in your picture showing -5 (g, I suppose)?

There's a reset button. Plus that one is OLD!

"Just about any"?  Are you excluding GA?

Exactly.              What's GA?

He's also excluding airliners. They don't have them either. The "just about any" part of this answer is incorrect.

Erik is asking for sensors, not gauges.

@jamesqf That unit has a lock feature (turned by a screwdriver on the back) for transporting the unit when not installed in an aircraft, protecting it from damage by holding all three needles against the bottom stop.

@user46893, if you don't know that GA stands for General Aviation you are definitely not qualified to make such a broad sweeping, (and un-substantiated) statement as you did in your answer.  When was the last time you were in a cockpit and saw such an instrument, and what type aircraft was it?

Some g-meters had max. & min. arms for quick reference.

@jamesqf because some manoeuvres involve negative g

@Pete Kirkham: Yes, I do realize that some maneuvers involve negative g (though I personally would not care for one that involved -5g :-))  But the unit is not in a maneuvering plane, it's being held in someone's hand, so it ought to be showing either 0 or +1 (depending on what you use as a reference)/

@jamesqf sorry, my misunderstanding

It has been answered for big aircraft, so just as a sidenote: Multicopters ("drones") have accelerometers (and other sensors) as integral part of their flight computer. Without it they would drop from the sky pretty quickly.


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RAT Deployment and Stowage
In this question, it states that the ram air turbine (RAT) deploys automatically on Airbus aircraft when there is a dual engine failure. Is there a way to stow the RAT in case systems are recovered and the RAT is no longer needed while the aircraft is airborne or does it have to be stowed by ground personnel? Are there differences between Boeing and Airbus aircraft insofar as deployment and stowage are concerned?

On the CRJs the thing is under the nose below the right seater's feet and runs a fixed freq ac generator. It's extended by a spring strut when both main ac buses are depowered. It's retracted by a hydraulic hand pump. I've done test extensions and the noise the thing makes is unbelievable. It sounds like a giant woodworking router under the floor.

@John K-Is the hydraulic retraction of the RAT on the CRJ done by the aircrew or ground maintenance personnel?

Normally by ground maintenance.  An airline crew would never do it in a million years anyway; they generally won't use the oil replenishment system either (you can top up the engine oil from a tank in the tail section).  But a corporate crew might, if they were stuck at a remote area and needed to stow it to get away. You need the Maint Manual procedure to do it.  Not sure if a maint log book entry is required if it's been deployed and restowed.

@JohnK I added a quote from the CRJ electrical description to my answer. It says "It is restowed manually, on the ground, by maintenance personnel."

The RAT is typically deployed using a spring loaded lever and cannot be stowed in flight. See this question for more details on deployment: How do Ram Air Turbines get deployed?
There is no fundamental difference between the implementations of Airbus, Boeing or Bombardier. I will quote some relevant sections of a few flight manuals here:

Airbus A320:

Ram Air Turbine (RAT)
A drop-out RAT coupled to a hydraulic pump allows the blue system to function if electrical power is lost or both
engines fail. The RAT deploys automatically if AC BUS 1 and AC BUS 2 are both lost. It can be deployed manually
from the overhead panel. It can be stowed when the aircraft is on the ground.

(Airbus A320 FCOM - Hydraulic Description, italic emphasis mine)

Boeing 777:

Ram Air Turbine (RAT)
The RAT, when deployed, provides hydraulic power only to the primary
flight control components connected to the center hydraulic system.
The RAT provides hydraulic and electrical power throughout the flight
envelope. In flight, the RAT deploys automatically if:

both engines are failed and center system pressure is low, or
both AC transfer busses are unpowered, or
all three hydraulic system pressures are low

The RAT can be deployed manually by pushing the RAM AIR TURBINE
switch. The hot battery or APU battery bus must be powered. The RAT is
deployed by a compressed spring. Once deployed, the RAT cannot be
stowed in flight.

(Boeing 777 FCOMv2 - 13.20.3 Hydraulics - System Description, italic emphasis mine)

Bombardier CRJ:

Air Driven Generator (ADG)
In the event of a complete AC power failure in flight, the ADG will automatically deploy
and supply emergency AC power to the ADG bus and to the AC essential bus. If the
automatic deploy function fails, the ADG can be deployed manually by pulling the ADG
manual release handle on the ADG CONTROL control panel at the rear of the center
console. [...]
The ADG cannot be restowed in flight. It is restowed manually, on the ground, by
maintenance personnel.

(Bombardier CRJ FCOM - Electrical - AC Electrical System, italic emphasis mine)

To stow the A320 RAT on the ground, the blue hydraulic system must be pressurized (via the electric pump) and the RAT EXTN SOL1 circuit breaker must be pulled. The RAT control panel is on the blue ground service panel. The RAT is stowed with STOW switch like this:

(YouTube: AIRBUS A320 RAM AIR TURBINE EXTENTION)

As you can see on the Airbus, stow is electrically controlled. The fundamental reason why retraction has to be done by a mechanic on the ground is they have to align the RAT blades with the orientation of the compartment.

@user71659 Well, if they really wanted to, they could install a mechanism that allows stopping the blades in the correct position and then retract it via a button in the flight deck. There is just no need for that.

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Can an MU-2 do a 360 degree roll?
If you wanted to do a Bob Hoover-like roll in an aircraft like the MU-2 that exclusively uses spoilers for roll control, I'm curious as to whether it can, and if so, what kind of control inputs would you use as the wings were near/at vertical?
I'm not a pilot, so I may be way off base here, but I can see the spoiler dumping lift off of one wing, causing that wing to drop, but once you approach wings vertical, then what?

You’d still have more lift from one wing than the other to produce the rolling moment, right?

Yes.
As long as the airplane maintains airspeed through the airmass it really doesn't matter what sort of control surface configuration is producing the rolling moment.  As long as the roll input is maintained, the aircraft will roll.  You can roll an airplane in a vertical climb or dive, with ailerons, spoilers, flaperons, etc., it really doesn't matter.

once you approach wings vertical, then what?

Presuming that the angle of attack remains positive, since the lift vector is not opposing gravity the aircraft will first begin to turn, and once past 90 deg AOB the lift vector will cause the nose to drop and the airplane to lose altitude.  Altitude loss can be compensated for my initiating roll with a nose up pitch, faster rate of roll, or wait until the you are upright again and pull back up to the horizon.

Now I get it. I was thinking that gravity caused the wing drop because of spoiler deployment, that's why I was having trouble understanding how the wing would start "up" when it was rolling from 90-270 degrees. It's all aerodynamics regardless of gravity. Thanks for the explanation!

@MichaelHall  That's for a postive G barrel roll.

@JohnK, what I described certainly applies to a barrel roll, but also to an "aileron" roll.  The question just asked about a 360 degree roll...

Yeah but if you're going actually into negative G, it's a whole new ballgame. The roll spoiler becomes a very skinny split flap.

that's why i worded my answer the way i did.

Depends on the type of roll.
If it's a barrel roll that maintains close to 1G "positive" throughout the roll (a barrel roll is more or less a loop stretched into a coil), the airplane doesn't know that the earth is revolving and the lift forces involved don't change (you remain in your seat with apparent gravity toward the floor), so around you go as normal.
An aileron roll, or slow roll, where you go into negative G flight with the wing generating lift up while upside down, it's another story. Once the wing is unloaded from positive G flight, the spoiler is now on the effective bottom (the high pressure side), acting more like a split flap. I would expect that at moderate extensions, the roll will continue when in negative G flight as the "split flap" spoiler is able to generate some lift in the direction of the roll (by simply deflecting air). With full spoiler (60 degrees), the rate could decrease as the "split flap" produces more drag than lift at high angles.  It's also possible that the chord of the roll spoilers being quite narrow, they do nothing but degrade lift in inverted flight same as upright flight, and you get a kind of control reversal.
I've never been able to find a description of how the MU-2 would perform in a slow roll or aileron roll, so it remains speculation.

I mean that's another way you could do a roll, but you can also do a fast aileron roll without pushing negative G.

You'll end up around zero G for much of it because you are going a bit ballistic to get it around quickly without going negative. WIth the wings unloaded, you are depending on the spoiler's ability to push the wing around as a deflector.

I've done lots of rolls in the spoiler equipped EA-6B, and yeah you might experience a little zero G, but the primary sensation is rotation.  I generally just left the stick neutral fore and aft.  Never tired a negative G roll, wish I still had access to one to try it!

So it sounds like the deflected spoiler acts like split flap, at least ta low AOAs. The MU-2's spoilers are actually cross connected like ailerons with a differential mixer. This means when you roll left, the left spoilers come out, and the right ones actually go down below the wing profile, about 1/3 the travel of the up spoiler. Mechanically, they are like ailerons, just skinny and moved forward ahead of the flaps.

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Why no CFM-56 for the E3?
Why did the USAF not upgrade the E3 from TF33 engines to the CFM-56 like they did the KC135? From the Wikipedia page on the E3, the latter variants sold overseas had the CFM-56 from the factory and had a considerable range advantage, so I'm assuming it would not take a great deal of engineering to retrofit the older aircraft, but maybe I'm wrong.

As mentioned in another answer to your question, the mission requirements for the two planes are different. The maximum takeoff weight of the KC-135 puts a limit not only on its range but also on the amount of fuel that it can carry to fuel other aircraft. The newer engines greatly increased this capability. According to a Boeing Frontiers article in July 2005 about the KC-135:

The re-engining program has increased the aircraft's fuel transfer capability by 50 percent and reduced fuel requirements by 25 percent.

For the E-3 the newer engines would mostly provide increased range and reduced fuel consumption. While this would be desirable, apparently the existing performance was considered adequate for the typical E-3 mission. Interestingly in the Wikipedia article on the E-3 it states:

As the test beds did not need the same 14-hour endurance demanded of the production aircraft, the EC-137s retained the Pratt & Whitney JT3D commercial engines, and a later reduction in the endurance requirement led to retention of the JT3D engines in production.

EC-137 being the development designation of the E-3, TF33 was the military designation of the JT3D.
The CFM-56 does provide quite a bit of extra range. Again from Wikipedia:

USAF and NATO E-3s have an unrefueled range of 7,400 km (4,600 mi) or 8 hours of flying.The newer E-3 versions bought by France, Saudi Arabia, and the UK are equipped with newer CFM56-2 turbofan engines, and these can fly for about 11 hours or more than 9,250 km (5,750 mi).

Apparently the U.S. felt that the existing range supplemented with aerial refueling was adequate, at least as in comparison to the additional cost and disruption of a re-engining program.

I think you hit on a key point with, "supplemented with aerial refueling." The E3 can refuel midair, while the KC135 cannot.

This answer is a good example of the delicate maths involved in operating the aircraft: how minor savings and improvements here and there scale up to massive improvements in certain areas of performance.

I do not have access to material that would definitely prove the following in the specific case of the E-3 Sentry, but:
As I understand the original USAF versions were delivered with T33 and the UK, France and other overseas versions had the CFM-56 from factory. Upgrades within the fleet always lead to temporary, possibly even long term complexity. Having the same Aircraft, the  E-3 in this case, run two different engine options will present challenges in logistics, maintenance and will also require extra some training.
Those and other possible considerations along with the bottom line, the overall cost of the upgrade, will be taken into account and weighed against the achieved performance gains of fuel consumption, thrust etc. when making the decision whether the upgrade is justified.
It is most likely that in the case of E-3 the overall equation did not justify the upgrade. In the case of the KC-135 the parameters of the equation would have been very different. The mission profile is, if not completely, at least substantially different from that of the E-3 which flies extended missions at stable altitude and mass, whereas the KC-135 flies much shorter missions within a much wider mass range.
The performance envelopes of jet engines of varying generations will be very different. The TF33 was probably deemed efficient enough in the use case of E-3, whereas the upgrade to CFM-56 offered large enough benefits across the varying loads required for the KC-135 operations.

To your point, from a Boeing Frontiers July 2005 article about the KC-135 re-engining, "The re-engining program has increased the aircraft's fuel transfer capability by 50 percent and reduced fuel requirements by 25 percent." A very different situation from the E-3 which would have just benefited from additional range and reduced fuel consumption. Which was likely desirable, but as you indicated the existing performance may have been adequate for the typical E-3 mission so the cost justification may not have been there.

@StevePemberton that would make an excellent answer! The extra range and endurance would not make much sense in the case of E-3 because of crew fatigue considerations for example.

At least, in the case of the USAF, this would be redundant, as the E-3 Sentry is going to be replaced by the Boeing E-7 AWACS aircraft.  Spending a great deal of money to re-engine the E-3 fleet then is just wasteful.

That's the situation now, but the KC135 started getting CFM-56 engines back in the 80's. My question asks why it was never done for the E3.

Besides the reasons given in this answer, there would have also been fairly large fixed costs in a re-engineering program which would be spread over all of the aircraft. In the case of the KC-135 a total of 420 aircraft were re-engined. In the case of the E-3 it would have been around 60. Maybe that wouldn't have been a major factor, but it was possibly part of the equation.

@RetiredATC I suspect, because, at the time, the Air Force felt that the JT-3D engines powering the E-3 did not merit change out as opposed to the far more inefficient J-57 engines, which powered  the original KC 135s


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What is the difference between azimuth and heading?
What is the difference between azimuth and heading?
I understand heading, track, radial, course and bearing but I don't know what azimuth is. I rarely hear it used. The Wikipedia definition is a horizontal angle measured clockwise from a north base line or meridian. How does that differ from heading?

Bonus question: when atc tells you there's a plane at your 2 o'clock, is there a word for that?

duplicate? http://aviation.stackexchange.com/q/8000/1467

@Federico Not sure it's a duplicate, but definitely closely related.

Not a duplicate because the other question does not mention azimuth, which is what I'm confused about. If somebody wants to edit azimuth into the other answers then close this as a dupe that would work

The word is relative bearing, a bearing relative to your heading. Azimuth is a bearing relative to North as @voretaq7 mentioned. An azimuth is a type of bearing.

@mins I thought relative bearing was putting up with your in-laws for the weekend :D

A heading (in the general case of moving "forward") is the direction your nose is pointed in. This may not be your course (as discussed here).
Headings are measured from onboard a traveling vehicle or object (e.g. from the cockpit of an aircraft or the bridge of a ship at sea).
An azimuth is a bearing, more precisely a compass bearing from a specific point of observation like a radar station. (The "North" used as a reference may be either magnetic or true depending on the system you're working with, but for purposes of this discussion it doesn't matter.)
Often the point of observation is fixed (ground radar, a control tower, an artillery spotter, etc.), though it need not be as long as the observation can be mapped to a compass bearing somehow.

Bonus Answer: The "O'Clock" positions are an informal type of relative bearing as mins noted: The 360-degree circle is broken up into 12 chunks (30 degrees each) for easier scanning, with "12 O'Clock" being directly in front of you and "6 O'Clock" directly behind.
An important caveat to be aware of is that when ATC provides a "clock bearing" for traffic it's relative to the aircraft's course (12 O'Clock is aligned with the aircraft's track over the ground as shown on the radar). Depending on wind there may be considerable variation between your course (direction of travel) and your heading (direction the nose is pointing), so the called traffic may appear in a different location when you look out your window.

So azimuth is from the POV of the radar and heading is from the POV of the aircraft?

@TomMcW Essentially correct: as you can see here the azimuth reported by the two stations is different, but the plane's heading is still roughly west-by-southwest: Heading is always relative to the vehicle you're on.

Sometimes ATC will use azimuth instead of clock if your airplane is maneuvering.

As already explained in voretaq7's answer, the true heading is the direction in which the nose of the aircraft points during a flight when measured in degrees clockwise from true north. Now consider you would see a star right ahead of you on the sky, the the azimuth of this star is identical to your true heading. Another example, if the star is 30 degrees to your right  on the sky and your true heading is 90° the azimuth of the star is 120°. The reference of the azimuth is True north in this case. The term azimuth is used in Astronavigation and when it comes to the use of navigation charts.

Azimuth is the horizontal angle in degree, with a reference point True North 0°. This reference point is fixed, cannot be changed
While Heading/Bearing is same horizontal angle in degree/mile, with a variable reference point (user can decide it) & it can be as well the True North

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text:
How does lowering altitude conserve energy in Solar Impulse 2?
In this article the author explains that the aircraft flies at a lower altitude to conserve energy.

During the day, it climbs to its maximum cruising altitude of 8,500 metres to capture the most rays. To conserve energy, it works its way down to 1,500 metres in the evening, and stays there overnight.

How is this possible?

I think what it is saying is that it starts a shallow descent to reduce power required, kind of like driving down a 7000 meter hill in your car.

Generally, best range is at high altitude(thin air, less drag), and best endurance is at low attitude(thick air, more lift) and low power.
It makes sense that at night they would have to descend to a best endurance altitude to conserve battery power.

Source: solarimpulse.com

"Working down" conserves battery power, as the other answers suggest. But it is also better for endurance to descend, simply because the flight Reynolds number will be higher, and the viscous drag lower.
See this answer for an explanation how the physics behind this works.
See this answer for the Reynolds number effect on a glider. Here the same aircraft is plotted at the same altitude, but with different wing loadings, so all typical speeds increase with wing loading, and so does the Reynolds number. You will see that L/D slightly increases with Reynolds number.
Also, the optimum polar point for minimum energy loss of propeller-driven aircraft is at a high lift coefficient $c_{L_{opt}}$, and this lift coefficient goes up with the wing's aspect ratio $AR$.
$$c_{L_{opt}}=\sqrt{3\cdot c_{D0}\cdot\pi\cdot AR}$$
At an aspect ratio of 20, this might be close to the airfoil's maximum lift coefficient, so flying slowly runs the risk of flying close to stall. Since the maximum lift coefficient goes up with the Reynolds number (after all, the viscous effects diminish and flow separation will be delayed), flying at a low altitude allows to make flying at the optimum polar point for minimum energy loss more safe or even possible.

From a non aero perspective battery efficiency is increased at warmer altitudes... (Conversely, as temperatures drop, the batteries become less efficient. And may require heating (or self heating).

@GürkanÇetin: You're right, this is certainly also a factor!

It is basically gliding at night. Not pure gliding but a powered glide - similar to what normal aircraft do on landing approach.
Altitude is energy. We all know this from science class - height = potential energy. Therefore an aircraft that's climbing uses energy and an aircraft that's gliding gains energy (technically it converts potential energy to kinetic energy).
Of course, it has to be done properly since the aircraft has no way of generating power at night. This is why it can't be a pure glide - if it loses too much altitude it won't have enough charge left in the batteries to climb.
The glide requires a lot less energy than maintaining level flight.
In effect, the aircraft uses altitude as a second type of "battery".

It is not "gliding at night". In the evening it descends to 1,500 meters and then stays there in level flight all night long.

Altitude is energy.  A long shallow dive, from 8,500 to 1,500 meters, recaptures the energy that it cost to climb up during the day.  It is completely analogous to charging the batteries with solar power during the day and drawing that energy out of the battery at night.

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text:
Does RTCA/DO-254 apply to power electronics?
DO-254 provides guidance for the development of airborne electronic hardware and it is recognized by the FAA in 2005 via AC 20-152 as a means of compliance for the design of complex electronic hardware.
Is DO-254 only applicable for electronics that perform some kind of 'logic' operations or is it also intended to be used to prove compliance for non-logic electronics, e.g. like power electronics.
Some may argue that power electronics as 'simple' but I believe todays electronic designs have to be considered fairly complex.

The description of AC 20-152 answers your question:

This advisory circular (AC) applies to manufacturers and installers of products or appliances incorporating complex custom micro-coded components with hardware design assurance levels of A, B, and C. These complex custom micro-coded components include application specific integrated circuits (ASIC), programmable logic devices (PLD), field programmable gate arrays (FPGA), or similar electronic components used in the design of aircraft systems and equipment. This AC provides a means (but not the only means) to gain Federal Aviation Administration (FAA) approval by showing the equipment design is appropriate for its intended function. Further, using this AC will help you satisfy airworthiness requirements when these types of electronic components are implemented

"Power Electronics" is a term so broad as to be nearly meaningless, but it doesn't matter: If the devices in question are designed to be installed on an airplane; have a DAL of A, B, or C (a failure would be Catastrophic, Hazardous, or Major respectively); and incorporate ASICs, FPGAs, or other circuitry with custom microcode then the FAA's guidance indicates you should comply with DO-254.

Note that anything you intend to install on an aircraft needs to demonstrate to the FAA's satisfaction that it is not hazardous to safety of flight in both normal and failure conditions, and that it does what you say it does.
Documented compliance with DO-254's guidance is one acceptable means of demonstrating the first of those elements (by showing robustness in the design, and graceful failure modes), though it is not the only means. (In other words if you're building a panel-mount cell phone charger or something equally ridiculous nothing indicates that you must comply with DO-254, but the FAA may ask you to comply if they feel your device is such that DAL A, B, or C is appropriate.)

I am not quite sure if this is really the answer to my question, beacuse AC 20-152 addresses only custom micro-coded components. It does not contain any comment regarding non-microcoded components as far as I knoe. For example the means of compliance for 14 CFR Part 25.1301 and 25.1309 can be provided in accordance with AC 20-152 for complex custom micro-coded components.

@Steffen I've added additional explanation, but with a term so vague as "power electronics" there's not much specific guidance we can give you: That can be anything from a panel-mount cell phone charger to a voltage converter for nav radios -- vastly different requirements. This is really pretty common sense stuff though: If your device could cause an emergency by failing you're probably going to have to comply with the guidance in DO-254 (or the equivalent) no matter how "simple" or "complex" it is.

Well, yes. So I also think DO-254 would be usefull to provide verification for compliance to FAR even for "non-logic" devices like power-electronics. Maybe DO-254 is not written for non-logic electronic devices but many artifacts could be adopted. I assume designing a circuit diagram with IGBTs, chokes, capacitors etc. may be interpreted to be the same as coding logic in VHDL.


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What material is the smoothest for air to travel over the wing?
Wings are pretty smooth these days and air travels over the wing easier, so what is the smoothest material

I don't see what's unclear about this question at all. What is the smoothest material? What material has the least skin friction (from airflow)? Those are two synonymous questions I could think of. If something about them or the OP is unclear, then at the very least a comment asking a specific question should be in order.

@DrZ214 This seems unanswerably broad to me. Is the question referring to the most smooth material ever used in a wing (which is almost certainly classified and/or proprietary and has as much to do with manufacturing process as with the material itself) or is it asking for the most smooth material ever fabricated (which may suffer the previous drawbacks in addition to almost certainly having other properties that make it unsuitable for aviation?)

@DrZ214 Engineers almost never ask "What material/device maximizes property x?" but rather "What material/device is good enough across all properties to get the job done and minimizes cost/benefit ratio?" Exotic materials and manufacturing processes are typically reserved for situations that actually require the properties of those materials, not because they have the maximum value for property x.

@reirab I don't see anything that broad. Both of your rephrasings are answerable, so it's certainly not "unanswerably broad". I'm sure I can find plenty of "beginner questions" on this site where the answerer took time to explain a few possibilities and answered each one.

@reirab Regarding your 2nd comment, I think you're just wishing for a different question or wishing that the OP was an engineer as opposed to just curious. "What is the smoothest material?" is a perfectly valid and clear question as far as I can tell, and does have relevance in aviation. Just because it's not all an engineer needs to know, doesn't make it bad, and certainly doesn't make it too broad. If anything it would be too narrow.

@DrZ214 In the present state, it is, at best, unclear which of those questions the OP was asking, if either of them. However, I would still argue that either of those questions is likely unanswerable for the reasons I already mentioned (i.e. answer to the former is probably not public knowledge and answer to the latter might not be public knowledge and is, at any rate, almost certainly not something used in aviation.)

@reirab I still cannot make much sense of your reasonings. Classified knowledge doesn't affect the question's relevance. You can add on "that we know of" to almost any question and it would be effectively the same. Whether or not the material can be used on aircraft does not affect the aviation relevance of the question. We would need to answer the question first, and even if an answer turned out to be "no, we can't do that" does not mean the question was irrelevant in the engineering process or aviation considerations...

...Voretaq7 did a not so bad job of answering this and it's not even that long (the pictures make it look longer). Let's not expect beginners to expound upon multiple possible engineering possibilities. That would almost be like answering your own question in the OP itself.

Well first let's dispel the rumor that wings are "pretty smooth" - ideally (in the dreams of aircraft designers and the mathematical perfection of fluid dynamics modeling) they would be, and they certainly are from a distance, but when you get up close and personal most aircraft wings are ugly, bumpy things (as you'll see below). Modern composite wings are a notable exception to this general rule, but there are a few other exceptions in the history of aircraft design.

The material is, for the most part, not relevant: We usually paint aircraft, so the smoothness of the underlying material contributes less to the surface than the paint preparation, clear coat, and wax.
What does matter are the construction techniques used. A few examples (in rough order of decreasing smoothness):

A modern composite wing can have an almost completely smooth surface (with no rivets or seams in the airflow) - for example a 787 wing, or this SR-20 wing:

Some aluminum designs use "bonded construction" (glue) with fewer disruptions of the wing surface. One example are the Grumman AA-1 and AA-5 series aircraft, which has a smooth wing except for the fuel caps and contoured seam lines:

Conventional aluminum wings can be assembled with flush rivets, which once primed and painted give a very smooth surface with only slight irregularities around each rivet:

(Please ignore the blue protective film, and open rivet holes. This is from an RV-14 under construction & was the best photo I could find.)
Fabric wings - one of the oldest wing coverings - are actually pretty smooth. They have "rib stitching" to attach them to the substructure, and are stretched taught over the frame (using heat & dope to shrink the fabric), but the overall result is a very smooth surface between the ribs, with some irregularity over them from the stitches:

Typical aluminum aircraft wing is attached with typical rivets - the head of which protrudes into the airflow slightly. You can see some rivet (and screw) shadows in this image:

Note that I've only considered the TOP surface of the wing (where airflow and smoothness is most critical). The underside of a wing usually has many other things sticking out into the airflow (fuel tank vents, access/inspection panels, hinges & tracks for control surfaces, etc.) which are a much greater disturbance to the airflow than whatever fasteners are attaching the skin to the rest of the structure of the aircraft.


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Can an aircraft or helicopter run into its own wake vortex?
I was recently flying my rc helicopter and I would sometimes turn it around and move it forward and sometimes I lost control of the helicopter while doing that a few times and crashed it everywhere (accidently crashed it into my poor cat). Every time I fly this helicopter it randomly starts turning to the left uncontrollably. Also I have had where I kept the thrust at the same level and then I would fly it into a different room and it would start raising in altitude uncontrollably. So can a helicopter or aircraft fly into its own wake and was it the wake of the helicopter that caused these problems with my helicopter?

Also, the very small size makes it very susceptable to air currents. A blowing vent, a fan on an electronic piece of equipment, or just air convection could cause something like fly it into a different room at it would start raising in altitude uncontrollably. Especially a ceiling fan.

Was the cat OK?

Late but a roll to one side during high speed flight might be retreating blade stall, but from the info in the question does not sound likely.

Helicopters are not easy to fly - whether full-scale manned aircraft or RC models. I would bet that your issues are more related to pilot error than any fault of the machine (other than the fact that it is a helicopter, and inherently unstable).
That said, almost any aircraft can be maneuvered in such a way as to encounter its own wake, and the helicopter is no exception. In fact it's much easier do to this in a helicopter than in a fixed-wing aircraft: A condition known as "settling with power" (or "vortex ring state") can occur when a helicopter descends into its own rotor wash, e.g. by descending directly vertically in calm air. The results of such an encounter can be "unpleasant" - a number of helicopter accidents have been attributed to this sort of encounter.

I think I ran into my own downwash. I would fly around in circles above my bed and that could be the cause of why I keep descending without control. That downwash from a little helicopter is so powerful. I didn't even think that a little helicopter like that could blow the sheets off my bed.

I've certainly run into my own wake turbulence when doing steep turns in a Cessna 172. In fact, it's the mark of a well-executed 360 degree steep turn that you do feel the bump of running over your own wake after coming full around. :-)


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How does CDI work when flying to VOR without DME?
Let's show what I mean on an example.
Flying towards a VOR (not knowing the distance), HSI course pointer pointing
   towards the VOR. Let's say the plane is really far from the VOR and we turn
   the course pointer 5 degrees (no matter what way) CDI will move to one side
   (far because of the distance). If the plane was close to the VOR and we turned
   the course pointer 5 degrees CDI would not move as much (or should not because the
   deviation is really small).
based on this example CDI needs to know distance from VOR to work properly, yet it works without knowing the distance. How?
(If I'm wrong about something, quickly correct me)

I suppose you are looking for something more specific than "The receiver does some magic and converts it into a number of dots of deflection on the indicator" : )

@fooot, I believe the key point in this question is what the VOR CDI indicates.

When the plane is sufficiently far from the VOR, you can determine distance without DME by setting a timer, turning perpendicular to the radial and reintercepting a different radial.

The VOR and ILS CDI indicates difference, in degrees, between the radial selected and the radial sensed.
If the plane was far from the VOR and we turn the course pointer 5 degrees, the CDI will move to one side. If the plane was close to the VOR and we turned the course pointer 5 degrees, the CDI would move exactly as much, because it indicates the angular difference in degrees.
This is in contrast to a CDI slaved to GPS, which indeed indicates lateral distance from the selected track in miles (en-route; GPS approach has to simulate ILS).
VOR does nothing with DME. DME is completely independent piece of equipment that was introduced later than VOR. The only relation is that there is a standard correspondence between VOR and DME channels and the DME receiver automatically tunes to the DME frequency corresponding to the VOR or ILS frequency tuned on your VOR/ILS receiver.

This explains alot... Thanks Just for clarification: I was thinking if you had VOR/DME then you know how far you are and CDI could show the derivation like the one slaved to GPS... But yeah... I've been expecting too much magic :)

@IsawU, on approach, the angle is what you want to see, because there you need the sensitivity to increase as you approach the runway. And en-route you just know that closer than some distance you just don't need to bother and just maintain heading for the few remaining miles.

Maybe this gif (https://en.wikipedia.org/wiki/VHF_omnidirectional_range#/media/File:VOR_principle.gif) will help you understand how VORs work a little better. Here you can see what Jan explained in his question. VOR work by measuring the time difference between two signals, one is a beam pulsating 20 times per second, the other rotates 20 times per second, showing the aircraft what course it is on seen from the VOR.

The change in "sensitivity" of the CDI needle as you approach a VOR has nothing to do with the DME distance - it depends on your lateral distance from the VOR.
You could be 5 miles straight up from the VOR (showing 5 miles on the DME), but the VOR receiver's sensitivity will depend on your lateral distance from the VOR.
This is because the VOR is displaying a difference in angular position: how many degrees of arc you are away from the desired course. The OBS knob on your CDI is selecting a roughly one-degree wide corridor that you want to be within, which is a pie-shaped wedge around the radial you've selected.
That wedge extends out from the VOR, getting wider as you get further from the station.
To illustrate, look at the 50-degree wide corridor highlighted in yellow below:

This has two practical sets of effects on VOR navigation:

The CDI needle sensitivity varies inversely with distance from the station

Far from the VOR station the CDI needle has low sensitivity: A large variation in position (miles) is a small number of degrees of arc away, so you could be a mile off the centerline of your selected course with the needle still nearly centered.
Close to the VOR station the CDI needle is extremely sensitive and "twitchy" - a tenth of a mile left or right could be full-scale deflection, because it represents being several degrees of arc off course.
VERY Close to the VOR station the CDI needle twitches erratically to the point of being nearly unusable because the VOR decoder in your aircraft can't determine which radial you're actually on. (Shortly after this you will usually fly past the station, and eventually the needle will calm down again.)

The OBS knob sensitivity appears to vary directly with distance from the station
As Jan noted an ideal VOR (transmitter and receiver) would vary the same number of dots of deflection for the same number of degrees dialed on the OBS, whether you're 10 miles from the station or 10 feet. With real world hardware though the OBS knob's sensitivity is also affected by distance from the VOR, but in the opposite way from the CDI needle.

Very Close to the VOR station the distance between each radial gets small enough that they are practically overlapping, and your radio may wind up decoding the VOR signal as indicating any one of those overlapping radials. Spinning the OBS knob appears to have less of an effect here, particularly with mechanical meter-movement CDIs, because the needle is twitching enough that what you're seeing is an average of what's being decoded.
Far from the station - far enough that the VOR receiver can definitively decode which radial it thinks you're on - the CDI appears to be more sensitive: The signal is definitely 065 degrees, and when you change the OBS value to 070 the needle will move to reflect the difference between the selected and detected value, with no erroneous decodings to mess it up. At these distances the VOR behaves more-or-less ideally, as Jan described.


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Unusual Traffic Ene 68 degrees over Dorset United Kingdom - No strobes, green or red just solid white lights
I have spotted a line of traffic 1 minute apart, at least 12 objects, with 3 objects to the left and right of the main line of traffic.  Clear night, no pollution, no noise, whit solid lights only.  I know what satellites look like in the night sky and that is exactly what this looked like in terms of height and speed but there was a stream of them equally distanced apart with 3 random ones to either side of the line of objects.  There was no strobe, the white light of each was showing form the front, side and rear. No green, no red.
I have never seen anything like this, nor have I ever posted anything like this but please if you know what this was let me know.  Can re positioning aircraft because of covid-19 fly with no strobing or colour lights etc?  Is it military?
I filmed this tonight 29th March over southern UK at around 2120hrs.  Many thanks.

Could it be a Starlink cluster?

You were most likely seeing satellites, the weather in the UK has been amazing for satellite viewing over the past week, even in London. If that's the case you're better off asking this in Space.SE. You need to post the direction the satellites were traveling, not just where they appeared from

Since you filmed it, could you post the video online somewhere and share a link to it? That may make it much easier for people to help answer your question. In general, though, unless they're going into combat, aircraft do not fly without navigation & anti-collision lights, no matter the purpose of the flight.

Satellites can look like this shortly after a serial deployment from a single launch vehicle.


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What is ATR Automatic Thrust Restoration in MD-83?
What is ATR Automatic Thrust Restoration  in MD-83 ?

The MD-80 series has two systems that can increase thrust in case an engine fails during takeoff:

Automatic Reserve Thrust (ART)
This system is independent of the throttle position and can increase the thrust of the remaining engine to the maximum rated thrust in case of an engine failure. It can be turned off with a switch on the glareshield, because it must not be used when using a FLEX takeoff thrust.
Automatic Thrust Restoration (ATR)
This system will use the autothrottle servos to increase thrust up to the go-around thrust setting after an engine failure.

Thrust Recoveries for Engine Failure During Takeoff
In takeoff mode, two separate systems attempt to provide maximum available
  thrust when the performance of one engine significantly differs from the
  other. These systems are automatic reserve thrust (ART) and automatic
  thrust restoration (ATR).
Automatic Reserve Thrust (ART) System
The ART system combines features of the digital flight guidance computer
  (DFGC) and the JT8D-200 fuel control to provide maximum rated thrust in
  the event of an engine failure during a normal thrust takeoff. Upon actuation
  of the ART system, thrust is increased without throttle movement by the
  opening of a solenoid-operated fuel valve in the engine fuel controls of both
  engines.
The ART system is READY when the airplane is on the ground, the ART
  switch is in AUTO, either slat is extended, both engines are operating at or
  near idle, and the ART system self-test is complete. The ART system is
  subsequently armed when the N1 on both engines reaches 64% RPM.
The ART system is actuated when the DFGC detects any one of the
  following: 30.2% differential in N1 RPM, invalid N1, DFGC failure,
  electrical power loss, or manual DFGC switching. Upon actuation, the ART
  system increases the EPR of the operating engine(s) from normal takeoff
  EPR to maximum takeoff EPR (an increase of approximately .05 EPR) by
  opening the solenoid-operated fuel valve in the engine fuel control. Once
  ART is actuated, the maximum takeoff EPR limit is displayed on the thrust
  rating indicator, and the EPR gauge. Once actuated, the ART system is
  latched (ART fuel valve remains open) until the ART switch is moved to OFF.
The ART has an automatic self-test feature. If the ART system fails the selftest, the ART INOP annunciation illuminates. Dispatch is allowed with a failed
  self-test; however, the ART switch must be moved to OFF to disable the ART
  system.
The ART switch, with AUTO and OFF positions, is on the upper instrument
  panel. Two annunciator lights are on the center instrument panel. The
  READY light indicates ART has successfully passed the self-test. The ART light
  indicates the system has successfully activated.
Automatic Thrust Restoration (ATR) System
The Digital Flight Guidance Computer (DFGC) automatic thrust restoration
  (ATR) is a feature, separate from the ART system, that increases thrust under
  certain conditions in the event of an engine failure during takeoff. Once
  activated, the ATR will unclamp the throttles (if the autothrottle system is
  engaged) and move both of them equally until one of the engines reaches the
  Go-Around EPR Limit.
The ATR is armed if:

The flight director pitch axis is in takeoff mode,
The airplane is above 350 feet radio altitude, and
Both engine EPRs are below the Go-Around EPR limit.

After arming, the system will activate if the differences between the engines
  are greater than or equal to 0.25 EPR and 7% N1 (in the same direction), or
  (for DFGC models -930) the airplane’s vertical speed decreases to less than
  zero for 5 seconds. In these cases, the throttles will unclamp (if the
  autothrottle system is engaged) and move to the Go-Around EPR limit.
If ART is armed and the ATR is activated, the EPR limit will be the
  maximum inflight takeoff rating reduced by the same amount that the ART is
  designed to provide. This correction prevents overboosting the engine if the
  ART system subsequently actuates.

(MD-80 FCOM Sec. 18 - Power Plant)

thankk you so much


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How does an airliner adjust and move exactly over the coloured lines before stopping in front of the jetways?
How does an airliner adjust and move exactly over the coloured lines before stopping in front of the jetways?

Closely related https://aviation.stackexchange.com/questions/25493/are-aircraft-still-guided-by-ground-marshals

I'm pretty sure there is another question which has details of several of the automatic gate systems, but I just can't find it...

There are a few different ways. Probably the most low-tech and widespread is by marshaller guidance, which is a person outside the aircraft guiding the pilots by means of visual signals:

The exact hand signals to use are defined by the ICAO, meaning that they are identical no matter where in the world you are flying.
Most larger airport have some sort of "docking system" at some or all of their stands. There are different types, but they are all external systems that provide visual guidance for the pilots, just like a marshaller. The simplest form is a big mirror next to the parking position:

More common are electronic docking systems, which use various sensors to detect the exact position of the aircraft and calculate any corrections needed by the pilots. They are placed directly in front of the position where the aircraft must stop, typically mounted on the terminal building or a high mast:

Also, most jetways have some degree of movement, so the plane doesn't have to be super precise, because the end of the jetway can then be moved up to the plane by ground crew before they open the doors.  (This is critical for situations where different types of aircraft may dock at the same gate.)

And when the pilots (or the marshaller) get it wrong, you then have to wait for a tug to pull the plane in. In some places like the cramped terminals in LAX, the aircraft actually stops quite a distance from its gate position and is pulled into its final position (using the same vehicles which are used for pushback).


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How many cabin crews are needed for an A350-1000?
How many cabin crews are needed for an A350-1000?

crews, one, as always. are you asking about crew members?

It depends where you are flying, i.e. which countries rules apply. In the US for example, the minimum cabin crew is determined by the seating capacity:

For airplanes having a seating capacity of more than 100 passengers—two flight attendants plus one additional flight attendant for each unit (or part of a unit) of 50 passenger seats above a seating capacity of 100 passengers.

(CFR 121.391 (a) (4))
The A350-1000 can seat up to 480 passengers, which would then require 10 flight attendants. Typical seating is however only 369 according to Wikipedia, which would only require 8 flight attendants.
In Europe, EASA defines the minimum number of cabin crew in the Type Certificate Data Sheet (TCDS):

For the A350-1000, the minimum required cabin crew number established during the aircraft
  certification process is 8, irrespective of the Maximum Operational Passenger Seating Capacity
  (MOPSC).
[...]
In accordance with the operational requirement ORO.CC.100-Number and composition of cabin
  crew, if the MOPSC for the specific aircraft exceeds 400, the minimum required cabin crew number
  becomes 9.

(EASA A350 TCDS)
So here 8 or 9 flight attendants are required, also depending on the seating capacity.

WOW thank you very much

What about the A350-900? Also most airlines put more flight attendants in because of the different classes. Sure by law it may be by capacity, but in an airline's perspective, that's not the case.

@AirCanada001 Typical seating for the -900 is 315, so 7 flight attendants would be enough in the US, but EASA still requires 8: "For the A350-900, the minimum required cabin crew number established during the aircraft
certification process is 8 (2 per exit pair)". Of course airlines can use more flight attendants, these are just the minimums for safety reasons, especially during an emergency evacuation.

@Bianfable: OK, thank's for the answer, you provided much more clarity.


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Bizarre photograph from an airshow
I ran into this picture on wikipedia, entirely by accident.

I recognise 121.5MHz as the emergency frequency, which seems apt for this particular photo.
This looks like two aircraft crashing into each other, but apparently it's from an air show and there's no mention of a crash. What's going on here? Put bluntly, it looks like the F/A-18 is humping a chopper. Are my eyes being deceived by camera magic, or is this an incredibly bizarre mid-air stunt?

…and that's why you avoid overlapping subjects when shooting with a telephoto lens.

While the print is 2D, the world is 3D. Is that enough for an explanation?

Not enough to justify a full answer but the “STBY 121.5” marking on the drop tank is for air defense intercept duty to notify pilots to tune their radio to Guard for further instructions.

This question seems to have attracted both positive and negative votes. That would suggest this is at least half decent as it is but some of us want to see it improved. Any feedback, as always, from the downvoters is welcome...

RE - "Not enough to justify a full answer but the “STBY 121.5” marking on the drop tank is for air defense intercept duty to notify pilots to tune their radio to Guard for further instructions. "  btw that has already been the subject of a question/ answer on this site before, possibly involving this exact photo, or at least an aircraft of same type in same markings.

Bit sad that this got downvoted so much, since it's a valid question even if the answer might seem obvious to those of you more familiar with aircraft photography. (I don't actually care, it just seems like poor practice for SE)

As was pointed out in the comments, this is a trick played on you by your eye and the telephoto lens used to take this picture. More on this phenomenon here:
Photograpy Stack Exchange - What does it really mean that telephoto lenses "flatten" scenes?
Your perception is not entirely without merit. Here is a comparison of side views of the two aircraft in question, Eurocopter Super Puma and F/A-18C:

While this picture might not be entirely accurate, it does show that the two aircraft are quite similar in size. One would think that the Helicopter is huge and the fighter jet is quite compact, but no. Throw into the equation the conversion errors form 2d to 3d inside our brain, and everything goes haywire.
The picture is form a situation where the two aircraft were actually flying in formation, this is a standard performance of Swiss air force, similar performance is shown in this video:
Youtube - AIR 14 Payerne F/A 18 Hornet, Super Puma

Freezing that video at 23s gives almost the same shot.  There's no confusion with the video rolling as your brain can interpret the relative distance

Whoa, a fighter and a helicopter flying in formation, something I had not seen before.

It's kinda tricky, as the helicopter and the jet will be operating at opposite ends of their speed range. Helo will need to go near max maneuvering speed, while the Hornet will have to fly at relatively high AoA and power setting to maintain level flight.

Which is why you see a slight nose down attitude on the helicopter and a very nose high attitude on the jet.

Thanks for this answer! The sizing was really throwing me, along with the weird lack of parallax (presumably because the telephoto lens distorts the parallax).

@Polynomial Parallax isn't what you think it is - there's no parallax in a still image. You need at least two images (or a video), whereby the linear shift of objects from one image to the next is inversely proportional to their distance from the camera. You may be thinking of depth of field.

@TypeIA oddly enough, parallax can effect non-stereoscopic observations: for example some rifle scopes have parallax adjustment, and the one on my high power scope clearly changes (also) the depth of field of the image when adjusted.

@Jpe61 It doesn't have to be stereoscopic, it just has to be moving. The parallax adjustment on a scope changes the focal point of the optics relative to the reticule. It does change the DoF, but if you take a snapshot from a particular eyepoint through the scope, there is no movement and therefore no parallax.

As @TypeIA implies, it's the parallax in the video that blocks the illusion (the word escaped me yesterday)

@TypeIA Poor phrasing on my part. What I meant is that the subjects being the same (apparent) size, being in equal focus (+ lack of DOF effects), and having no clear difference in motion blur or aberration, all contribute to a distortion of the sense of depth when parallax is lost. Photographs usually retain some clues (e.g. defocus, relative motion blur) that help the eye resolve depth in the absence of actual parallax, particularly in a moving scene, but it seems like telephoto lenses tend to reduce those effects (at least in skilled hands) and obfuscate the depth.


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Is it legal to hire a test pilot to conduct the first flight of a homebuilt ultralight in the United States?
Is it legal in the United States to hire a test pilot to conduct the first flight(s) of a homebuilt ultralight? This seems like a bit of a grey area- FAR 103.1(b) states that an ultralight aircraft is a vehicle that “is used or intended to be used for recreational or sport purposes only”
but I don’t see any reason that a pilot could not be paid to evaluate the flight characteristics of the aircraft.

Is this the first flight of a (factory built) ultralight? Or the first flight of a homebuilt ultralight?

A hypothetical home built ultralight.

It is legal as long as they are paid in bushels of wheat, or salmon.

If there is some kind of legality, don’t pay him for the flight. Pay him for his consultation before and after the flight.

The FAA highly encourages you to find or hire a test pilot for your E-AB.
Advisory Circular 90-116

This advisory circular (AC) provides information and guidance on the
Additional Pilot Program (APP) for flight testing experimental aircraft. The APP was developed to improve safety by enhancing Builder/Owner Pilot (BP) skills and mitigate risks associated with Phase I flight testing of aircraft built from commercially produced kits through the use of a qualified additional pilot and powerplant testing. The APP is an optional program which provides another pathway to conducting Phase I flight testing. The traditional option for a pilot to test their aircraft solo during Phase I is not covered or affected by this AC, and remains an option for those who choose to do so in accordance with their aircraft’s operating limitations.

Further on:

16. COMPENSATION. Compensation for the services of a QP or an OP may only be accepted or provided in accordance with the privileges and limitations specified by certificates held by the individuals.

This means in order to accept compensation, the test pilot must have at a minimum, a commercial license, appropriate medical, and be qualified to fly the aircraft. I'm not entirely sure if recent court rulings require a LODA though, you may want to check with your local FSDO.

It looks like that document only applies to aircraft which have an "Experimental certificate issued under § 21.191(g), Operating
amateur-built aircraft, or § 21.191(i)(2), Operating light-sport aircraft," but the question is about part 103 ultralights, which do not have airworthiness certificates.


text:
How to fly for the United Nations?
I can't find anything online about flying for the United Nations, either helicopter or fixed-wing, and any of their aviation programs, and I'm really interested in this. Can someone point me toward some more information about flying for the UN?


text:
How does a fighter pilot calculate approach/landing speeds?
Let's say an F/A-18C returns to land after expending some payload. Fuel burned and weapons released made the plane lighter. I guess the aircraft computer can tell the pilot the actual weight of the fighter on one of the displays. With this information, does the pilot use a chart or computer to calculate the correct speed for landing or is there a rule of thumb?

The "easy" way seems to be to fly by AoA, unless something goes wrong with the sensor.  What many pilots do is to see what the plane does at a given weight and airspeed (descend, level, or climb) based on elevator backpressure.  One might imagine consistent weight helps keep AoA safe, whereas widely varying weight may require an on-board flight engineer and/or computer.

F/A-18 pilots (really, most Navy pilots, but I'll adhere to your example) fly approaches and landings by AoA, not airspeed.

How does a fighter pilot calculate approach/landing speeds? On modern aircraft you simply keep the VVV (Vertical Velocity Vector) within given approach parameters by the FCS (Flight Control System), e.g. follow the "E bracket" on an F/A-18C Hornet or the classical generic "C bracket" (sometimes "I bracket") on most other aircraft. Keep an eye on your dedicated AOA (Area Of Attack) indicator as a safety net. Oftentimes it's even available audible. :^)

In the F-4, we had a base speed to use for each aircraft landing configuration, to which we would add 2 knots for each 1000 pounds of fuel. If we still had any significant external stores hanging on the plane we would treat as that as additional fuel.
But when we were actually flying the approach, we had an aural tone in our helmet (headset) that told us exactly what the AOA was, it went to a steady tone when we were at optimum landing (approach) AOA.

Exactly the same in the Prowler, minus the aural tone.  You flew the USAF version?  Because the USN version likely had indexers on the glareshield.

We had the indexers on the glareshield as well, as well as a clock-style instrument on the panel, but the aural tone obviated the need for anything else. We used the aural tone to maintain optimal AOA during close in Basic Fighter Maneuvering (BFM) air combat.


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What specific features of the Sukhoi 27 make it a rival to the F-15?
I just read an article about the Sukhoi 35 that mentioned as a side note that:

The Soviet SU-27 was designed to kill the American F-15

Of course the political context of the Cold War agrees with that.
But are there specific features for which the Su-27 was such a "killer"?
My understanding of Soviet airplanes of the 1980s is that:

The MiG-29 was made as a light fighter to give numbers to the VVS and for interception
The Su-27 is made to be a multirole plane and is heavier than Mig-29
Older airplanes are kept for specialized tasks (SU-24, Su-25...)

I would expect the more heavy SU-27 to be designed to shoot at American bombers, and the lighter and more agile (?) Mig-29 to attack enemy fighters such as the F-15. Am I correct? What features make the Sukhoi 27 an F-15 killer?

And the article's authoritative source on that statement? Maybe include a link to the article...

The Su-27 is supermaneuverable, the F-15 is not. That alone is a huge advantage, but BVR weaponry makes that almost a moot point. It all comes down to pilot skill. A skilled Eagle driver could kill a Su-27 in BFM, and a skilled Sukhoi pilot could kill an F-15 BVR. It's a very complex issue considering the number of iterations there now are of each model.

@RonBeyer There was no source for this statement in this article. It was only mentionned in the intro, the rest of the article being about SU 35 ans F 15 concurrent sellings

Principally the SU-27 with its Phasotron N001 pulse Doppler radar provided the Soviet Air Force with a look-down/shoot-down capability designed as a counter to the American B-1 bomber.  But it also had to be designed to counter the newer generation of American fighters designed around the Energy Maneuver philosophy of aerial combat.  The Kremlin came to the conclusion that it, too, wanted a high-low mix of aircraft with the Sukhoi design being selected as its premier interceptor, which could provide a BVR capability plus close in maneuverability on par with high end America fighters like the F-14 and F-15.  The Mikoyan Gureyvich bureau was responsible for the development of a small lightweight fighter, principally designed to counter the threat of low end, cheap and maneuverable NATO aircraft like the F-16.  That airplane became the MiG-29/-35 series of fighters.
I dunno if you would call a SU-27 an “F-15 killer”, though it was designed - at least in theory - to match it.  Given what I’ve seen of the Russians in action in Ukraine as of late, I’m not personally impressed with at least their conventional capabilities.  The Su-27 is inferior to the F-15 in terms of engine tech (Russian and Chinese engines are about 20 years or so behind modern western jet engines), mission systems and radar.  Russian and Chinese guided missile tech is inferior to western weapons as well, taking the teeth out of the Flanker compared to the Eagle or the Tomcat.
The airframe, however, is surprisingly good, and follows a very pragmatic and functional design philosophy typical of Russian military hardware.  For a ‘Teen Series’ fighter, the SU-27 airframe does offer a lot of advantages:

It holds a lot of gas, giving it long legs to fly BARCAPS or fly deep into enemy territory on OCA missions, etc.
High thrust to weight ratio offers good acceleration, climbing and sustained turning capabilities.
Blended wing-body fuselage plus Leading Edge Root Extensions (LERX) vortex generators offer a low overall wing loading combined with excellent sustained high alpha maneuvering capabilities.
The fly by wire systems along with an advanced air frame design offers good post-stall maneuvering capabilities, seen in modern variants like the Su-30, -35 and -37.
The passive IRST capability combined with BVR IR guided missiles, helmet mounted cueing systems and all aspect off bore sight AA-11 missiles give it a big stick as well.

There are a lot of very enticing capabilities associated with the SU-27.   But the limited engine tech plus inferior electronics tech handicap it compared with western equipment.  It is a generation out of date compared to modern western fighters like the F-22A.
UPDATE:  While I say that an SU 27 is not in the same league as American fighters, it would be an intriguing sight to see an SU-27 given a radome-to-tail overhaul and upgrade using western technology e.g. western jet engines, new cockpit, new mission systems and modern western weapons. In that fight, and against an F-15 piloted by an equally capable or lesser driver, the Flanker would be formidable adversary.

The SU-27 does not really have long legs though they can hold lots of jet fuel. It's engine is horribly inefficient. My country fly modified variants - SU-30MKM and they have roughly half the range of our F18. Maneuverability is awesome though so if you manage to get tangled up in a classic dogfight with it you're in trouble

"It is a generation out of date compared to modern western fighters like the F-22A" which means it can be an excellent "F-15 killer", or?

@slebetman Negative.  On internal gas alone, a hornet has about half the range and combat radius of a Flanker (~1000 NM vs ~2000 NM).  The SU-27 also has a ~900 NM combat radius, something an F-18 can only dream of.  Second BFM (dogfighting) is the fundamental and ultimate form of air-to-air combat.  Only a fool would dismiss it as over.


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How do we renew an aircraft's expired FAA registration?
How do we renew an expired aircraft registration? It needs repair and is not airworthy. The loan on the aircraft was paid off. We may need to sell the aircraft and transfer the title. How is the registration related to the title?

The FAA's registration site says (emphasis in the original):

If aircraft registration has expired and a renewal certificate has not been issued, received, and placed in the aircraft, then the
aircraft is without authority to operate.
[...]
If a renewal application was first received by the Registry after the expiration date, the renewal process no longer applies. An
aircraft owner may apply for registration of an unregistered aircraft
under 14 C.F.R. §4731(a), by filing an Aircraft Registration
Application, AC Form 8050-1, the $5.00 registration fee, and evidence
of ownership (if it is not already on file at the Registry).

In other words, it's effectively a new registration. The title could be used as the evidence of ownership.
You can find form 8050-1 here.


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Do turbine engines contain engine oil and fire-extinguishing fluid?
When aircraft turbine engine companies sell their engines to aircraft manufacturers, does the product contain the tank and pipe for engine oil and fire extinguishing fluid in the same package? Or is it something that the aircraft manufacturer has to build for the engine independently?

For turbofan engines used on passengers aircraft, oil system and fire sensor system are part of the engine. This is not the case for fire extinguishers which are manufactured by third parties and installed by aircraft and nacelle manufacturers at the proper locations. The nacelle is the aerodynamic engine housing with the inlet, fan duct and various systems.
In the picture below, showing a final assembly line of a GE factory, the black tank on the fan case is the oil tank:

Source
Oil tanks and pipes
They are part of the engine because they are deeply interlinked with other engine systems (bearing sumps, gearboxes). If you see a technician on the ground opening some panel on the engine during the aircraft turnaround, it's likely to check the oil level in the tank:

Top image, CFM56-7B engine: source.
Bottom image: CFM56-7B Component Identification Manual
Fire protection system
For fire protection includes the fire detection system and the fire extinction system. Detection, comprising temperature sensors (fire loops) embedded in the engine, is part of the engine. In the image below, #263 is a sensor loop around the HP turbine at the rear of the engine:

Image source: CFM56-7B Component Identification Manual
The fire extinction system is composed of gas bottles external to the engine, located in the main wheel wells or the fuselage.

Image source
The gas has been a halon until 1994 (production stopped to protect ozone) and is now a hydrofluorocarbon. The gaz is released from its bottle using a switch in the cockpit:

Halon bottles, Boeing 737 NG, source
The gas is then conveyed from the bottle to the engine by pipes running in the fuselage, the wing and the engine nacelle. There is at least one bottle for each engine, and there are other bottles for cargo holds and various protected locations.

Your question has been expertly answered in great detail.
Consider this: Engines of all types are made of metal parts that are spinning against each other. Metal spinning against metal will quickly result in the parts quickly welding themselves together and seizing if they are not separated and protected by a thin coat of oil.
Due to the protective lubricating properties of oil you can think of oil as being the "life-blood" of any engine.
Considering that, if an engine company were to produce and deliver an engine without an included oil system, they would essentially be assuring the engine's immediate destruction (or, at a minimum, counting on other engineers who were not involved in designing the engine to design and add an appropriate engine oil system).
It would mean that the engine manufacturer had not finished building the engine.

Then entire question has been expertly answered in great detail. I cannot match the current exhaustive explanation of the fire extinguishing system.
I was thinking that if the person who asked the question is brand new to engines, I would tell him a little about why the oil is so extremely important that ANY engine produced without an oiling system would be an incomplete unfinished engine that wasn't ready to be delivered to anyone- not even an aircraft manufacturer.
EDIT: Oh, I see that you are mins!  Your explanation is fantastic and exhaustive- complete with photos.  Great job!


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Why are there (almost) no civilian single engine jet aircraft?
I have been looking at various engine configuration in jet powered aircraft. Most have 2 jet engines mounted either side of the fuselage, there are trijet models (e.g. B-727, MD-11) there are also 4-engine aircraft (A380, B-747) and there were even 6-engine (e.g. A-225 Mriya). Yet when looking for a single engine jet powered aircraft, the only one I could find is the Cirrus Vision SF50, which is a fairly new model.
There are, of course, various military/fighter models - but I am more interested in civilian aircraft at this point.
Is there a particular reason why there aren't (almost) any single-engine jet powered aircraft?

Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on [meta], or in [chat]. Comments continuing discussion may be removed.

I'm not sure why most of the answers/comments talk about airliners as the question did not specifically asked about them, but about civil aircraft in general.

Is there a particular reason why there aren't (almost) any single-engine jet powered aircraft?

I'm going to assume that the term "jet powered aircraft" refer to turbojet or turbofan engines and not to turboprops as there are plenty of single-engined turboprop aircraft.
Structurally
Having a single jet engine is challenging as there are very few  feasible locations for engine placement, each comes with its own issues.
If you put the engine inside the fuselage, it means it needs to be in the rear.
This means the entire central fuselage must be entirely or almost entirely hollow to allow for the air intake/s. This of course does not leave a lot of room for passengers nor cargo.
If you put the engine above the fuselage:

The front part of the fuselage might block/shadow the air flow to the engine in higher angles of attacks.

It is probably going to be a maintenance challenge to reach the engine.

Cabin noise insulation needs to be very good when you have a jet engine a few inches above your head.

Most likely requires a V-tail (or H-tail) if you don't want the vertical stabilizer inside the engine exhaust.

Financially
Turboprop engines are much more efficient in the flight regimes applicable for a small civil single-engined aircraft than "pure" jet engines.

@RTO Indeed it is. Not sure which part of my comment your comment contradicts

"Much more efficient:"  a recent flight report of a BD-5J, from the point of view of a prop pilot, said as soon as it's running, you're out of fuel and on fire.

Counterexamples abound. The 727, Tristar, Falcon 900EX, 7X/8X, and Yak-40 all put the engine "in" the fuselage, i'd say the fuselage wasn't hollow. The HS-121 put 1 engine in the fuselage, and half an engine in the shadow of the fuselage. The DC-10/MD-11 put the engine above the fuselage, and it's not a V- or H-tail. If you can't the obvious answer is a T-tail, but you got the Falcons with the cruciform tail, or a canard.

Some others I missed: Tu-154, Yak-42 are all s-ducted "in" fuselage. Falcon 50 belongs with the other Falcons. With single-engine prototypes, the PiperJet did it DC-10 style. The Diamond D-Jet was the most unique by mounting the engine under the fuselage and had a split duct.

There are two reasons to want to power your aircraft with a jet engine: either "haha cool jet engine" (which is a super valid reason) or you want to go well over 300 knots in order to shorten travel time. Going 350 knots instead of 300 knots only really makes a difference in travel time if you are going far, far enough that a big fraction of trips will cross oceans. Crossing a body of water on a single engine is spooky.
(The PT6 turboprop engine is so reliable that people are comfortable flying them across the atlantic on single engine planes- in fact, this is how TBMs are delivered to the states. Jet engines of a suitable size for a single engine airplane, with that level of reliability, don't really exist)

Why would a turbofan engine be less reliable than a turboprop? You've deleted the gearbox and variable pitch propeller parts. The rest of the engine runs on the same concept.

There's no reason for turbofans in general to be less reliable than turboprops in general. The PT6 turboprop engine specifically is extremely reliable

To be legally operated by a passenger airline, its airplanes need to comply with certain regulations. One of those stipulates what should happen in case of an engine failure:

§ 25.121 Climb: One-engine-inoperative.
(a) Takeoff; landing gear extended. In the critical takeoff configuration existing along the flight path (between the points at which the airplane reaches VLOF and at which the landing gear is fully retracted) and in the configuration used in § 25.111 but without ground effect, the steady gradient of climb must be positive for two-engine airplanes, and not less than 0.3 percent for three-engine airplanes or 0.5 percent for four-engine airplanes, at VLOF and with—
(1) The critical engine inoperative and the remaining engines at the power or thrust available when retraction of the landing gear is begun in accordance with § 25.111 unless there is a more critical power operating condition existing later along the flight path but before the point at which the landing gear is fully retracted; ...

This obviously cannot be achieved with a single engine. The engine failure case is considered too likely and its consequences too severe to allow single engine airplanes to legally carry more than a handful of paying passengers. In other words: Redundancy. This philosophy can be seen in many aspects of passenger airplane design.

Passenger airlines can operate under Part 135 using single engine aircraft. Surf Air has scheduled operations using PC12s.

@Pilothead That's why I said "more than a handful". Have you ever seen a single engine airplane with more than 12 passenger seats?

@PeterKämpf  " Have you ever seen ..." --> No. but if a cheap and cost effective one was available Ryan Air would fly them :-(.

@RussellMcMahon


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Average weight of fuel required to carry 1Kg additional cargo weight per hour
I'm looking for an average number here, I know there is not one correct answer as there are many parameters to take into account.
Alternatively, I could also use something different, like : how many extra fuel does it take to carry 1kg extra freight from city A to city B.
I guess there is more chance to find empirical data than a perfect answer.
I've found many articles explaining that airlines can save XX€ by removing YY item from the plane, or that it's worth replacing crew books by ipad etc...
And it makes total sense that less weight = less fuel required.
But I've not found any details that would allow me to calculate what I need.
Basically, I want to be able to estimate if it's worth replacing an item by a ligther but more expensive one in an aircraft.
The only thing I found so far is this :
Weight of fuel required to carry 1Kg additional weight per hour : 0,04  Kg
But I've no idea where it comes from, so I'd appreciate if you could confirm and give some detail.
Ultimately, my goal is to get airlines to switch to ligther stuff so they emit less CO2.
Thanks a lot!

Related: How much does it cost extra to fly with one additional passenger?

To give more context, my company manufactures cargo pallet nets.  So ideally I'd like to get an average figure that would work for a B747 400 full freigther fully loaded. My goal being to calculate how much fuel can be saved by replacing the 39 PMC nets by ligther products and see how much can be saved over a year of operation.

OK, so if I understood well the other post, a good average is 0.005 gallon per extra pound, so about 0.042 liter per extra kilogram, this is quite big! It means about 60 USD saving per year each time you remove 1kg of weight

I answered a similar question on Physics.SE: Does it take significantly more fuel to fly a heavier airplane?


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See-through/transparent aircraft
I am a pilot who loves the aviation experience. Can we build a full see through plane using material as strong as the current materials used to build the body of an aircraft. Is there research or work being done on this?
I think that aviation should see through the glasses of adventure travel. And, we should be able to see it all as we fly. Is this possible??
Thank you for your time and excitement,
jason

This isn't possible currently. Best thing you can do if you want this experience is fly a tube frame ultralight.

Would make using the restroom on a commercial flight... awkward...

...looking for traffic.

Wonder Woman had a plane like that in 1942

It is possible, but not using typical modern construction techniques.
You have to build a structural frame and then clad it with a non-structural skinning. This technique was widely used in the early days, although the skinning was doped fabric and not transparent.
The frame technique is still used for many ultralights and hang gliders, and some transparent films are strong enough to be used for the skinning. But these types seldom have a significant fuselage. One exception I can think of is Paul MacCready's man-powered Gossamer Condor and Gossamer Albatross prize-winning man-powered aircraft, which had a fuselage nacelle slung under the wing, containing pilot Bryan Allen and the drive train.
Some helicopters such as the Bell Sioux have such large "fishbowl" cockpit glazing that it exposes all the cockpit innards. The Edgley Optica was a prototype observation aircraft along similar lines.
Talking of cockpit glazing, some flying wings had a prone pilot with the glazing forming part of the wing leading edge. These included some of the Northrop and Horten designs.
The Junkers G.38 airliner accommodated some passengers in wing cabins and the glazing to these formed the wing leading edge.
So yes it can be done in principle. But materials such as plexiglass or mylar film are not really suited to such things as enclosing engines or resisting birdstrike, while strengthened glass or polycarbonate get heavy and expensive so that on most planes, only small windows are practicable.
Advanced materials such as structural glass are being increasingly used in architecture, and I have no doubt that the manufacturers and researchers will keep pushing the envelope to develop new materials and new applications. But I don't know of any research focused on airframes.

I'm surprised that you forgot to mention the Linke-Hoffman R.I picture link which naively tried to cover the fuselage and tail in Cellon to make it transparent.

Certainly it's possible, since - as you yourself say - many ultralight aircraft are built this way, and use modern construction techniques.  I think the problem is that you are confusing aircraft with commercial airliners.

As Ron Beyer has said, it is currently not possible. Research is being done to create transparent materials of great strength. So far, the cost of producing this material would far outweigh its usefulness in aviation. It is more likely that you will see the application of this technology in cars before aircraft. It’s safety would have to be well proven and documented. Plus, weight would be less of a factor in automotive engineering.
Plus, the engine and cockpit would still need to be made from non-see-through material. This would defeat the purpose of making the fuselage transparent. Your view would still be obstructed.
There are much easier ways to experience adventure aviation. Tube-frame ultralights, hanggliders, paramotors, and other currently produced aircraft would all accomplish the task of flying with an almost totally unobstructed view. Many helicopters like the Schweizer S300 would be the closest to aircraft to what you are probably looking for.


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On an aerofoil or circular cylinder why does the airflow stay attached after passing the point of maximum thickness?
Why does it follow the curvature rather than separating?
Its not due to the coanda effect as the accelerated flow near the surface of the aerofoil can't be considered a jet. It's not due to surface tension because there is no interface between two fluids. I know the flow of a fluid is due to pressure, inertial and viscous effects.

Is it due to viscous effects on fluid particles in the boundary layer?
Is is due to the pressure normal to the surface of any body submerged in a fluid?
Another reason?

Thanks for your answers!

Would this answer help in answering your question? In short: It is suction caused by local curvature and incidence.

@PeterKämpf - Yes that does help, especially the part about viscosity meaning that air molecules, because of their oscillation, tend to assume the speed and direction of their neighbors. When combined with the velocity profile in the boundary layer this could explain the continued attached flow. I'm not so sure about your statement that if the flow didn't follow the curvature then a vacuum would form and so it reluctantly follows the curvature. When flow separation does occur a vacuum isn't formed, we have a turbulent wake.

There is another answer on flow separation here. Separation only happens when the flow at the surface is reversing.

@David, there can't be stagnant air near the receding surface because that would mean higher pressure, so it would get sucked out. So there can either be recirculation, or flow along the surface. Because viscosity prevents the air from turning too sharply, the recirculation is only possible at high angles (stall) while at low angles the flow stays attached.

This is due to the Coandă effect.
https://www.britannica.com/technology/fluidics#ref129652

In the 1930s Henri Coandă, a Romanian scientist, described what is now
known as the Coandă effect, a major contribution to fluidic
technology. He observed that as a free jet emerges from a jet nozzle
the stream will tend to follow a nearby curved or inclined surface. It
also “attaches” itself to and flows along this surface if the
curvature or angle of inclination is not too sharp. Coandă explained
this tendency as being caused by the jet stream’s entraining (picking
up) nearby fluid molecules. When the supply of these molecules is
limited by an adjacent surface, a partial vacuum develops between the
jet and the surface. If the pressure on the other side of the jet
remains constant, the partial vacuum, which is a lower pressure
region, will force the jet to bend and attach itself to the wall.

This is only partially true. The Coandă effect is for a jet in otherwise stagnant fluid, but in this case all the fluid is moving, so it is not applicable as defined. The entraining mechanism is there, and prevents pocket of stagnant air from forming near the surface. This still allows both attached flow and a flow from behind filling in, the later being what happens in stall. Viscosity prevents the later at lower angles – viscosity prevents the air from taking sharp turns, which is what fixes the rear stagnation point to the trailing edge.


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What are the abandoned technologies in aviation industry
And technologies has come to end of to their life cycle  Especially technologies that developed for aviation. This could be a manufacturing technology or an engine rather than avionics or sensors.

The biggest one that comes to mind for me is Cathode Ray Tube technology for cockpit displays.  Rockwell Collins (called something else now) has ceased production of CRT displays and users of Proline CRT Primary Flight Displays, Secondary Flight Displays and EICAS systems are in a scramble to replace the CRTs when they wear out.  There are a number of companies offering LCD units to upgrade CRT units under STC and this is happening across the airline industry.
Another one could be navaids like Non Directional Beacons that are starting to go the way of the A/N Range and the related receivers, and eventually other analog navigation equipment.
Mechanically, not that much.  Light aircraft are still built today with magnetos that use points and condensers like tractors from the 40s.  They are simple and work.  The business is very conservative, a vital self-preservation characteristic in such a life-or-death environment, and things that are simple, work, and are easy to fix and easy to diagnose, and let you know when they are degrading, are hard to replace.

Venturi tubes to drive vacuum gyros.

One could say the vacuum drive as such is abandoned, but the current C172S still has it.
Corrugated skin.

Photo by Stefan Krause, from Wikimedia

Of course, we need to exclude vintage and replica aircraft: one could probably find some flying with any technology.

I was thinking more navigation related - ADF, NDB, LORAN, VOR getting there.
Incandescent light bulbs going away, being replaced by LEDs.
Vacuum pumps & vacuum driven gyros going away too.

Bulbs etc. are still plentiful in older flying aircraft, and can't always be replaced with LEDs. But when it comes to navigation, one could mention astronavigation (with sextant etc.) or even 'land and ask locals where you are' (no joke, it was happening in the early days).